In thermal infrared (IR) sensors (e.g. bolometers), absorber coating is generally applied to convert IR incident power into temperature change. Different absorber materials have been investigated (see, e.g., thin metals and SiN films, as disclosed in F. Jutzi, D. H. B. Wicaksono, G. Pandraud, N. de Rooij, P. J. French, “Far-infrared sensor with LPCVD-deposited low-stress Si-rich nitride absorber membrane—Part 1. Optical absorptivity,” Sensors and Actuators A 152 (2009) 119-125.). One exemplary material that has been used for dedicated single-use applications, such as space missions, is gold-black, as disclosed in D. A. Bell: A Geostationary Earth Radiation Budget Instrument, Proc. SPIE 2209.236.1994. Gold-black is the most common special case of nano-structured porous coatings known as “metal-blacks”.
Gold-black material can achieve absorption close to unity over a broad spectral range from visible to far IR, as disclosed in L. Harris, “The Transmittance and Reflectance of Gold-black Deposits in the 15- to 100-Micron Region,” J. Opt. Soc. Am. 51, 80-82 (1961); and L. Harris and P. Fowler, “Absorptance of Gold in the Far Infrared,” J. Opt. Soc. Am, 51, 164-167 (1961). Gold-black is an nano-crystalline deposit of gold with extremely low density and thermal mass and a refractive index close unity, as disclosed in W. Becker, R. Fettig, A. Gaymann and W. Ruppel, “Black Gold Deposits as Absorbers for Far Infrared Radiation,” phys. stat. sol. (b) 194, 241 (1996).
Broader commercial application to array detectors has been hampered by gold-black's extreme mechanical fragility, which also makes it difficult to pattern. For array detectors, it is desirable to coat only the sensing element to avoid thermal and electrical bridging between pixels. Laser ablation has been used in the past to remove coatings deposited between the pixels, but this slow process is unsuited to mass production, as disclosed in N. Nelms, J. Dowson, N. Rizvi, and T. Rohr, “Laser micromachining of goldblack coatings,” Applied Optics Vol. 45, No. 27 (2006).